1. Field of the Invention
The present invention relates to improvements in the performance of steam turbines, and particularly to a steam turbine including nozzles, buckets, a casing, a turbine rotor, and a hydrophilic coating portion disposed on such components to prevent erosion, and also relates to a hydrophilic coating material used for the hydrophilic coating portion of the steam turbine.
2. Related Art
Steam turbines convert the energy of high-temperature, high-pressure steam generated by a boiler into rotational energy by supplying the steam to cascades of nozzles (stationary blades) and buckets (moving blades). FIG. 9 shows the mechanism of a power-generation system based on a steam turbine.
Steam generated by a boiler 1 is further heated by a heater 2 and is introduced into a steam turbine 3. Referring to FIG. 10, the steam turbine 3 includes a turbine rotor 4 implanted with buckets (moving blades) 5 circumferentially and a casing 13 supporting nozzles (stationary blades) 6. The buckets 5 and the nozzles 6 are combined into stages 7 arranged in the axial direction of the turbine rotor 4.
The steam introduced into the steam turbine 3 is expanded through a steam path 8. The turbine rotor 4 then converts the energy of the steam at high temperature and high pressure into rotational energy. Referring back to FIG. 9, the rotational energy is transferred to a motor 9 connected to the turbine rotor 4. The motor 9 converts the rotational energy into electrical energy.
The steam, which has lost its energy, is discharged from the steam turbine 3 and is introduced into a condenser 10 which cools the steam with a cooling medium 11, such as sea water, to condense it into water. A feed pump 12 feeds the water to the boiler 1 again.
The steam turbine 3 is composed of a high-pressure turbine, an intermediate pressure turbine, and a low-pressure turbine, depending on the temperature and pressure of the steam to be supplied. For the power-generation system described above, particularly, a wet steam having a moisture content of about 10% flows near the final stage of the low-pressure turbine because the steam temperature has decreased. In a steam turbine used for a nuclear power plant, a high-pressure turbine stage operates with wet steam because saturated steam is initially supplied.
When wet steam flows through the nozzles 6, moisture contained in the steam condenses into water droplets on the nozzles 6. These water droplets are combined into coarse water droplets which are scattered by the steam flow and collide with the buckets 5 downstream of the nozzles 6. The collision tends to damp the torque of the buckets 5 and thus decrease the total performance of the turbine 3. The coarse water droplets also contribute to erosion of surfaces of the buckets 5. The erosion decreases the thickness of the buckets 5 and thus shortens the life thereof. This problem has increasingly become serious as longer blades are used for final turbine stages.
Practical operation of the buckets 5 and the nozzles 6 with wet steam will be described in detail.
FIG. 11 is a schematic diagram illustrating the behavior of wet steam and water droplets in the vicinity of blade cascades operating with the steam.
Moisture 14 contained in the steam is mainly deposited on concave surfaces of the nozzles 6 to form water films 15. The water films 15 appear significantly in the middle of the nozzles 6 and flow to the trailing edges thereof while increasing in thickness. The water films 15 are released from the trailing edges of the nozzles 6 into a main stream (operating steam) as water droplets 16. If the water films 15 are thick, large and coarse water droplets are released.
FIG. 11 illustrates a velocity diagram showing the difference in behavior between the steam and the coarse water droplets 16. According to the velocity diagram, the coarse water droplets 16 released from the trailing edges of the nozzles 6 have a velocity lower than the steam and thus flow to the buckets 5 at a velocity higher than the steam. In addition, the coarse water droplets 16 collide with the buckets 5 at an angle that is significantly different from the angle of the steam. Therefore, the collision of the coarse water droplets 16 causes significant erosion at the tips of the buckets 5, particularly on the convex side thereof.
FIG. 12 is a schematic diagram illustrating the behavior of the water droplets 16 on the concave surfaces of the nozzles 6.
In practice, the water films 15 do not appear uniformly on the nozzles 6, but appear locally as water film flows 17. The thickness of the water film flows 17 increases at an accelerating rate as they gather to the trailing edges of the nozzles 6. The water film flows 17 are released into the main stream as the coarse water droplets 16. This release does not occur uniformly over the height of the nozzles 6, but occur locally and irregularly. The coarse water droplets 16 thus result from the localization of the water film flows 17 and the formation of extremely thicker water films at the trailing edges of the nozzles 6 than other portions thereof.
FIG. 13 is a graph showing the relationship between the size of the water droplets 16 and the amount of erosion of blades attacked by the water droplets 16.
Referring to FIG. 13, the amount of erosion increases at an accelerating rate as the size of the water droplets 16 increases, and this means that prevention of the formation of the coarse water droplets 16 is effective against erosion.
FIG. 14 is a graph showing the distribution of wetness along the height of the blades.
According to FIG. 14, the centrifugal force of the rotation of the buckets 5 causes a large amount of water droplets to gather to outer portions of the buckets 5 while the swirling of the steam flow causes a large amount of water droplets to gather to outer portions of the nozzles 6. The water film flows 17, and thus the coarse water droplets 16, appear significantly on the outer portions of the buckets 23 and the nozzles 6.
One of the techniques for reducing the effect of wet stream is the removal of the coarse water droplets 16 from the steam flow. Japanese Unexamined Patent Application Publication No. 8-210105, for example, discloses a drain catcher provided by forming a suction outlet in a casing accommodating buckets to recover water droplets with the aid of the centrifugal force of a steam flow. In addition, Japanese Unexamined Patent Application Publication No. 7-42506 discloses a slit nozzle capable of sucking water droplets through a slit formed in a surface thereof. Such techniques have been put to practical use.
Referring to FIG. 15, furthermore, grooves 18 may be formed on the buckets 5 so as to extend in the radial direction. These grooves 18 can cause the water droplets 16 to flow and fly to the outside of the buckets 5 so that a drain catcher 19, for example, can recover the water droplets 16. The effect of wet steam can also be reduced by increasing the intervals between the nozzles 6 and the buckets 5. This technique decreases the size of the water droplets 16 and facilitates the acceleration thereof to reduce the velocity of the collision of the water droplets 16 with the buckets 5.
On the other hand, Japanese Unexamined Patent Application Publication No. 10-280907 discloses an example of a technique for inhibiting erosion of blades. According to this publication, a hard coating or material with excellent erosion resistance is applied to the ends of the leading edges of blades. For example, Stellite, a cobalt-based alloy, is used for 12Cr alloy steels, and a Ti-15 Mo-5 Zr-3 Al-based alloy coating is used for titanium alloys.
Among the above techniques for reducing the effect of wet steam, the removal of water droplets requires a significantly complicated removal mechanism and thus increases production cost. The formation of openings in a steam path or an additional structure on blade surfaces to remove water droplets can cause aerodynamic loss leading to decreased turbine performance. In addition, it is difficult for water droplets to selectively remove and discharge from a steam, and such techniques undesirably discharge part of the steam which would otherwise generate work together with water droplets, thus lowering turbine efficiency.
Increasing the intervals between the nozzles 6 and the buckets 5 not only can cause aerodynamic loss, but also can result in larger mechanical dimensions. This imposes serious design limitations in terms of ensuring reliability because, for example, a longer turbine rotor can cause larger vibrations.
The application of a hard coating or material with erosion resistance requires a sophisticated welding technique, and blazing can decrease the strength of blade substrates. In addition, hard materials are expensive, and sufficient reliability cannot be ensured in terms of, for example, adhesion strength because the use of longer buckets is associated with the collision of water droplets at higher velocities.
Thus, any of the known techniques for reducing the effect of wet steam can trade off turbine performance, reliability, or cost for the reduction of the effect of wetness, and a solution to that problem is urgently necessary.